Internally conducttvely coated



Jan. 31, 1956 J. H. CAMPBELL 2,733,371

INTERNALLY CONDUCTIVELY COATED DISCHARGE LAMP Filed May 12, 1950 lnvenlrors John H. CampbeLL,

His A tforneg.

2,733,311 Patented Jan. 31, 1956 INTERNALLY CONDUCTIVELY COATEDDISCHARGE LAMP John H. Campbell, Painesville, Ohio, assignor to GeneralElectric Company, a corporation of New York Application May 12, 1950,Serial No. 161,621 6 Claims. (Cl. 313-185) The present invention relatesto gaseous electric discharge devices generally,

relatively low resistance.

It has become relatively common to provide metallic stripes orconductive coatings on the outside glass walls of electric dischargedevices. For instance, this is done with respect to cathode ray tubesand some types of dis charge lamps. In general, however, the purpose ofthe coating is simply to dispel static charges or accumulations ofelectrons on the walls, so that high ambient humidity will not preventthe lamp from starting at normal voltage.

It is known that if a transparent conductive coating is provided on theinside of the glass walls of a low pressure positive column and theblast made considerably smaller. Although some improvements in thatdirection have been made in the past, there is as yet no commerciallyavailable lamp wherein the starting voltage is as low as the voltagerequired for stabilizing the discharge.

I have found that there appears to be an optimum range of resistance forthe internal coating of a gaseous discharge device which is neither ashigh as the resistance merely sufiicient to dispel electronicaccumulations nor as low as a metal stripe, the equivalent of whichmight A be a wire internally disposed within the lamp. In general, theoptimum resistance appears to be that which, when traversed by thecurrent suflicient to produce a glow discharge in the lamp, is justsufficient to produce a potential gradient along the glass envelopewhich is slightly in excess of the gradient required within the lamp toionize its inert gas constituent.

I have also found that a lamp provided with an internal conductivecoating lends itself advantageously to combinations of multipleelectrodes wherein a main electrode is provided is the provision of Forfurther objects and advantages and for a better I the coating resistanceduring the understanding of my invention, attention is now directed tothe following description and to the novel will be more clearly pointedout in the appended claims.

In the drawing:

Fig. l is a side view of an improved electric discharge lamp structureembodying my invention and including a schematic diagram of an operatingcircuit therefor.

Figs. 2, 3, and 4 are pictorial representations of consecutive stages inthe transition from a glow to an arc discharge within a lamp embodyingmy invention.

Fig. 5 is a side view of an electric discharge lamp having an internalconductive coating and provided with an improved cathode structure inaccordance with my invention, and including a schematic diagram of anoperating circuit therefor.

Referring to Fig. l, the device 1 comprises an elongated tubular glassenvelope 2 having sealed into the ends thereof a pair of electrodes 3and 4. These electrodes may consist of coils of tungsten wire coatedwith activated electron-emitting materials such as barium and strontiumoxides. The ends of the coils are brought out externally to the envelopethrough suitable seals in the glass in order to permit the applicationof a heating current to raise the activating materials to anelectron-emitting temperature. The glass envelope contains an inert gassuch as argon, krypton, neon, or mixtures thereof at a pressure of a fewmillimeters of mercury and also a small quantity of mercury which,during normal operation of the lamp, exerts a pressure of a few microns.

The inner wall of the glass envelope 2 is coated with a fine layer ofconductive materials such as, for instance, one of the metallic halides.I found that stan' nous chloride, which may be applied by vaporizationwhile the glass is at a temperature of approximately 550 0., provides asuitable transparent coating which hardly affects the light-transmittingproperties of the glass. In practice, after the transparent conductivecoating has been deposited on the glass, it is further coated with asuitable phosphor as in the usual fluorescent lamps.

Although internal transparent conductive coatings in electric dischargedevices have been suggested before, prior investigators'have apparentlynot made use of the fact that heated electrodes in combination with arelatively low resistance inside coating will provide a maximum loweringof the starting voltage. Past practice has indicated that use of aconductive coating outside or inside the bulb will merely dispel staticcharges and thus prevent hard starting due to high ambient humidity. Ihave found that, in general, the optimum resistance value is that which,when traversed by a current just sufiicient to produce a glow dischargewithin the lamp, produces a voltage drop along the longitudinal axis ofthe lamp giving a voltage gradient slightly in excess of that requiredfor ionizing the inert gas within the lamp. Naturally, the

In manufacture it is desirable to keep the resistance range to closerlimits in order to allow for changes in various heat treatments.

3 The 40-watt lamp, may, therefore, require a range of 50,090 to 150,000ohms.

Referring to Figs. 2, 3, and 4, the progress of the glow dischargewithin such a lamp has been illustrated in order to. facilitatecomprehension of the different manner of establishing an arc in a lampprovided with an internal conductive coating such as I have described.The glow discharges illustrated refer actually to those occurring whenthe lamp is operated from a direct current source, since the phenomenamay be more easily understood thereby. The only difference when the lampis operated from an alternating current source is that the character ofthe discharge alternates from one end of the lamp to the other at thefrequency of the voltage supply, so that the character of the glow atboth electrodes appears to be the same. In fact, however, the type ofglow established at each electrode during each half cycle of thealternating c rrent wave is identical with that established continuouslywith a direct current supply, so that, with these reservations in mind,the phenomena occurring with a direct current supply only need beconsidered.

It will be assumed that an operating circuit has been provided for alamp such as illustrated in Fig. 1, which comprises a discharge currentlimiting reactor 5 inserted in series between the lamp and a pair ofinput terminals 6, 6 adapted to be connected to a source of voltage. Afilament heating transformer 7 comprises a primary winding 8 and a pairof secondary windings 9 and 10 which are connected to electrodes 3 and4, respectively. This circuit obviously is intended for use with analternating current supply. However, those skilled in the art will haveno difficulty in conceiving a similar circuit for operating the lamp ondirect current, as will be assumed in the explanation of the characterof the glow in Figs. 2, 3, and 4 now to follow.

Referring to Fig. 2, after the electrodes have been heated to anelectron-emitting temperature and a voltage is applied across them toestablish a gradient within the discharge space of the lamp, thereoccurs initially a weak glow in the immediate vicinity of both thepositive and negative electrode. If the magnitude of the applied voltageis increased slightly, the glow at the positive electrode assumes aconical shape, the apex of which is pointed in the direction of thecathode. The lamp at this stage has a positive resistancecharacteristic. I surmise that this is due to the resistance of theinternal coating and is not actually inherent in the discharge. It wouldappear that at this stage of the glow, in the immediate vicinity of theelectrodes the current is carried by electrons and ions, but in thespace intermediate the electrodes, where no glow has developed as yet,the current is flowing through the conductive coating. The reason forwhich there is an optimum potential gradient along the resistance pathwill now become apparent in that it is necessary that the glow dischargefill the whole space between the electrodes before the transition intoan arc discharge can occur. For the glow to fill the. intermediatespace, it is necessary that the gas molecules therein be ionized andthis will naturally be assisted if the voltage gradient produced alongthe walls of the envelope by the current flowing through the resistivecoating is sufficient to cause such an ionization. If the resistance istoo high, the flow of current through the coating will be unnecessarilyrestricted and ionization in the vicinity of the electrodes will behindered. If the resistance of the coating is too low, the current fromthe glow discharge flowing therethrough will produce a voltage gradienttoo low to be of any substantial help in the ionization of the gaseouscolumn.

When the voltage applied across the electrodes is increased slightly,the apex of the conical glow emanating from the anode stretches out inthe direction of the cathode. In Fig. 3 the apex of the glow has reachedapproximately the mid-point of the lamp, and in Fig. 4 the, apex hasreached the glow which surrounds the cathode. Fig. 4 illustrates acondition which is generally unstable, as, the'glow discharge willshortly thereafter tain activating processes in. manufacture only,

spread out to fill the whole tube, and the transition to the arc.discharge will then occur with the formation of a hot spot on thecathode or negative electrode.

As an example of the advantages ensuing from this type of lampconstruction, the starting voltage required for 40-watt fluorescentlamps provided with argon at 3 mm. pressure has been reduced from 450volts to 270 volts in the instant start type and from 208 volts to 150volts in the switch type preheat ballast. Although this voltage is inexcess of the operating voltage drop across such a lamp, which is in theneighborhood of volts, it is well within the voltage which is usuallyprovided for regulating the discharge through the lamp in order tocompensate for its negative resistance characteristics. Thus, commercial40-watt fluorescent lamps are usually operated with a transformer andseries choke coil or with a high reactance transformer which has an opencircuit voltage of approximately 208 volts, which voltage is decreasedduring operation, as a result of the internal reactance drop, toapproximately 100 volts. A lamp having an internal resistive coatingsuch as I have described will start instantly, in such a circuit,without any preheat switching apparatus whatsoever provided a preheatcurrent is applied to the. electrodes.

Although I have described the. advantages of an internal transparentcoating with respect to electrodes which are preheated at starting, myinvention lends itself equally well tothe lowering of the startingvoltage with lamps wherein the electrodes are not preheated at starting,such lamps being commonly known as the instant start type. Without anymodification whatsoever, except the provision of an internal coatinghaving a resistance of the same order as that which has been described,a substantial lowering, of the starting voltage occurs.

I have found that a discharge lamp containing a low resistance internaltransparent coating lends itself particularly Well to an improved typeof electrode, as illustrated in Fig. 5. The lamp 20 therein is providedwith main electrodes 21 and 22 which may consist of coiled coils oftungsten wire thickly overlaid with activated electronemitting material.Both ends of the filaments of electrodes 21 and 22 are, brought outexternal to the glass envelope of lamp 20. However, this is for thecarrying out of cerand the upper terminal alone is utilized duringoperation. No preheat is required. so that a high reactance transformer23 without any; filamentary heating windings may be utilized foroperating the lamp. In association with electrodes 21 and 22, I providesmall auxiliary electrodes 25 and 26, respectively. These electrodes mayconsist of a simple coil of finer tungsten wire than utilized in themain electrodes 21 and 22, these auxiliary electrodes being also coatedwith. activated.electron-emitting material and con nected to the freeends of the main electrodes. They are physically displaced from the mainelectrodes and preferably located closer to the envelope wall than themain electrodes. As with the free ends of electrodes 21 and 22, the freeends of electrodes 25 and 26 are brought out external to the glassenvelope in order to facilitate the manufacturing process.

In operation, when voltage is applied across the electrodes, ionizationoccurs and surrounds the electrodes at both endsof the lamp. At thismoment, the current flows between the electrodes and the conductivecoating in their immediate vicinity. Auxiliary electrodes 25, and 26,being constructed of finer wire, have a lower thermal capacity and alsoa higher resistance than the main electrodes 21 and 22. As a result ofthe current, they heat up more quickly and arrive more readily at atemperature at which sufficient electrons are emitted. to permit thetransition to an arc discharge. milliamperes, in the case of a 40-watt,48-inch lamp. When the arc discharge occurs, the current becomessubstantially greater; and since, it must flow through the mainelectrodes, connected in series with the auxiliary electrodes, it causesthese main electrodes to reach electron-emitting The current above maybe as low as 70:

temperature. Thereupon, the arc shifts to these main electrodes, that is21 and 22, which electrodes are large enough to support the dischargewithout overheating, and normal operation ensues. It might be mentionedthat the hot spots forming on the electrodes always tend to shift towardthe end of the electrode closest to the point of application of theexternal potential causing the discharge. This is due to the fact thatthe voltage drop produced by the resistance of the electrodes betweenthe point where the hot spot occurs and the connection to the lead-inwire produces a voltage diiferential which tends to shift the hot spottoward the lead-in wire. During the life of the lamp, as the activatedmaterial is slowly destroyed in the immediate vicinity of the lead-inwire, the hot spot gradually works away from the lead-in wire toward theother end of the cathode until finally the lamp has reached the end ofits life.

While certain specific improvements have been shown and described, itwill, of course, be understood that certain modifications may be madewithout departing from the invention. The appended claims are,therefore, intended to cover any such modifications coming within thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric discharge device of the low pressure positive column typecomprising an elongated vitreous light-transmitting envelope containingan ionizable medium including an inert gas at a pressure of a fewmillimeters and a small quantity of mercury, a pair of main coiledfilamentary thermionic electrodes, activated with electronemissivematerials, sealed into opposite ends of said envelope, lead-in wiresconnected to one side of said main electrodes to provide currentterminals, a pair of auxiliary pressure posivitreous lightrespectivelyconnected to the other side of said main electrodes and physicallydisplaced therefrom, and a low resistance transparent coating of ametallic halide covertrodes and physically displaced therefrom, and alow resistance transparent coating of stannous chloride coveringsubstantially the whole inside suiface of said envelope.

4. An electric discharge device of the low pressure positive column typecomprising an elongated vitreous lighttransmitting envelope containingan ionizable medium including an inert gas at a pressure of a fewmillimeters and a small quantity of mercury, 21 pair of main coiledfilamentary thermionic electrodes, activated with electronemissivematerials, sealed into opposite ends of said envelope, lead-in wiresconnected to one side of said main electrodes to provide currentterminals, a pair of auxiliary coiled filamentary activated electrodesof substantially lower thermal capacity than said main electrodes andrespectively connected to the other side of said main electrodes andphysically displaced therefrom, and a low resistance transparent coatingcovering substantially the to raise said auxiliary electron emittingtemperature.

5. An electric discharge device of the low pressure positive column typecomprising an elongated vitreous lighttransmitting envelope containingan ionizable medium including an inert gas at a pressure of a fewmillimeters and a small quantity of mercury, a pair of main coiledfilamentary thermionic electrodes, activated with electronemissivematerials, sealed into opposite ends of said envelope, lead-in wiresconnected to one side of said main electrodes to provide currentterminals, a pair of auxiliary coiled filamentary activated electrodesof substantially lower thermal capacity than said main electrodes andrespectively connected to the other side of said main electrodes, said6. An electric discharge device of the low pressure positive column typecomprising an elongated vitreous light-transmitting envelope containingan ionizable medium including an inert gas at a pressure of a fewmillimeters and a small quantity of mercury, a pair of main coiledfilamentary thermionic electrodes, activated with electron- ReferencesCited in the file of this patent UNITED STATES PATENTS 1,980,534 Kirstena- Nov. 13, 1934 2,038,049 Kirsten Apr. 21, 1936 2,042,147 FairbrotherMay 26, 1936 2,064,369 Biggs Dec. 15, 1936 2,291,965 Jahncke Aug. 4,1942 2,297,454 Berger Sept. 29, 1942 2,306,925 Aicher Dec. 29, 19422,429,420 McMaster Oct. 21, 1947 2,441,831 Moore May 18, 1948

1. AN ELECTRIC DISCHARGE DEVICE OF THE LOW PRESSURE POSITIVE COLUMN TYPE COMPRISING AN ELONGATED VITREOUS LIGHT-TRANSMITTING ENVELOPE CONTAINING AN IONIZABLE MEDIUM INCLUDING AN INERT GAS AT A PRESSURE OF A FEW MILLIMETERS AND A SMALL QUANTITY OF MERCURY, A PAIR OF MAIN COILED FILAMENTARY THERMIONIC ELECTRODES, ACTIVATED WITH ELECTRONEMMISIVE MATERIALS, SEALED INTO OPPOSITE ENDS OF SAID ENVELOPE, LEAD-IN WIRES CONNECTED TO ONE SIDE OF SAID MAIN ELECTRODES TO PROVIDE CURRENT TERMINALS, A PAIR OF AUXIALLIARY COILED FILAMENTARY ACTIVATED ELECTRODES OF SUBSTANTIALLY LOWER THERMAL CAPACITY THAN SAID MAIN ELECTRODES AND RESPECTIVELY CONNECTED TO THE OTHER SIDE OF SAID MAIN ELETRODES AND PHYSICALLY DISPLACED THEREFROM, AND A LOW RESISTANCE TRANSPARENT COATING COVERING SUBSTANTIALLY THE WHOLE INSIDE SURFACE OF SAID ENVELOPE. 